Fertilized eggs, mouse brains, primary cells and established cells are generally used as sources for the production of vaccines. However, such traditional vaccine sources have several problems. For example, when it is intended to use fertilized chicken eggs for vaccine production, there are difficulties in raising chickens, managing the fertilized eggs depending on vaccine production schedules and purifying ingredients derived from egg proteins.
Fetal calf serum is generally added as a growth factor for cell culture. However, serum is susceptible to contamination with prions and viruses and its commercial products might vary in quality. Moreover, high-quality fetal calf serum products derived from Australian and New Zealand calves are highly priced, resulting in an increase in production cost.
MDCK cell lines are established cell lines where various kinds of viruses can proliferate. Since MDCK cell lines exhibit a strong tendency to attach to other surfaces, large-area culture vessels and carriers are required for large-scale culture, incurring considerable costs. Specifically, investment costs for culture equipment or carriers are vast and a processing step is needed to remove cells attached to carriers, posing the risk of loss of and damage to the cells. Thus, there is a need for a cell line that can be prepared by serum-free culture and suspension culture and is thus suitable for use in the production of a vaccine through animal cell culture in an economical and safe manner.
U.S. Pat. No. 6,825,036 discloses an MDCK-derived cell line that can be grown in serum-free culture and in suspension culture without a solid carrier. The U.S. Patent employs two approaches for the adaptation of the cell line to suspension culture. According to the first approach, the MDCK-derived cell line is obtained by direct suspension culture in a spinner flask. In this case, the cell density does not reach 1.0×106cells/ml, a level necessary for industrial-scale production. According to the second approach, the MDCK-derived cell line is obtained by culturing the original MDCK cells in the presence of beads as carriers, expanding the culture scale and growing the cultured cells in the absence of carriers. The second approach has the disadvantage in that the procedure is relatively complicated.
There are several reports that original MDCK cell lines are tumorigenic. Thus, there exists a danger of potential tumorigenicity when original MDCK cell lines are used for vaccine production. Under these circumstances, there is a need to develop a novel cell line that can be prepared by serum-free culture and suspension culture and preferably has very low tumorigenicity or is non-tumorigenic.